A mobile phone has to handle events, such as incoming network carried events or just “local” status changes in an application within the phone, and provide information to the user for him to respond to or reflect upon in one way or another. It can be everything from incoming calls, new messages, alarms and calendar reminders to network status and battery information.
Some of those events are considered critical enough to demand user attention and prompt the user with an alert, such as the pop-up alerts used e.g. for incoming calls and new messages in mobile phones.
Other events are considered less critical and are handled in other ways, e.g. changes in level of network coverage. In certain phones these changes are communicated to the user via a status bar at the top of the display.
How critical an event is depends on what the user is doing at the moment and the status of the telephone. For example, if a call is in progress there is usually no pop-up alert but only a symbol is shown in the status bar.